1. Field of the Invention
The present invention relates to a drive power source of a liquid crystal display in an image forming apparatus, that is, an LCD power source control method for controlling a drive voltage and a control circuit thereof, and an image forming apparatus having the control circuit.
2. Description of the Prior Art
Hitherto, in an image forming apparatus of a copying machine or the like, for example, there is one in which fine toner particles are used to copy an image of an original document onto a copy paper.
According to this image forming apparatus, the image of the original document is optically read by an image reading device and then the read image is formed as an electrostatic latent image on a photoreceptor such as a photosensitive drum. Fine toner is adhered to the electrostatic latent image of the photoreceptor so that the image is formed (developed) by the toner.
Moreover, in the image forming apparatus, a sheet feed roller draws a copy paper from a paper tray or the like and the drawn copy paper is conveyed to a position of the photoreceptor by a number of sheet feed rollers, whereby the image formed on the photoreceptor is copied on the copy paper. The copy paper having the image copied thereon in such manner is transferred by the sheet feed roller to a portion of a fixed roller which will be heated by a heater. After being heated by the heat of the fixed roller, the copy paper is then discharged therefrom. The image transferred on the copy paper is fixed thereon due to heating.
Such an image reading device, a sheet feed roller, a photosensitive drum, fixed roller, and the like are operated by a power source circuit that is provided in a main body of the image forming apparatus.
Further, various operation buttons or switches are provided on an operation panel in such an image forming apparatus, and an LCD (liquid crystal display) for displaying the present situation of the apparatus and the displays corresponding to the operation is normally provided as display means therein as well.
A liquid crystal display such as an STN-LCD (Super Twisted Nematic Liquid Crystal Display) is often used for the operation panel. A drive voltage from a power source circuit is applied to a logic circuit, whereby operation control of a liquid crystal display such as the STN-LCD is performed based on a timing signal generated from the logic circuit. For example, a frame signal is used as the timing signal. A low voltage drive power source of a +5V system, a +3.3V system, or the like is necessary in order to control the operation of the logic circuit, and a drive power source of a relatively high voltage such as +24V or xe2x88x9212V is necessary for driving the liquid crystal display as well. It should be noted that, in order to discern the low voltage drive power source of the logic circuit from the drive power source for driving the liquid crystal display, the drive power source for driving the liquid crystal display is explained herein as a high voltage drive power source.
These drive power sources are provided in an LCD module, that is, inside the module of the liquid crystal display, whereby the drive voltage of the logic circuit and the liquid crystal display may be increased and generated inside the module.
Since the power source circuit for driving the image reading device and the movable parts such as the sheet feed roller, the photosensitive drum, and the fixed roller is disposed in the main body of the apparatus as mentioned above in the aforementioned image forming apparatus. The present situation is that a low voltage drive power source for the LCD module is provided in the power source circuit of the main body of the apparatus in order to reduce costs.
Further, it is necessary to set input and interruption timing of the power source of the main body of the apparatus as follows in the image forming apparatus in order to prevent deterioration of the liquid crystal of the liquid crystal display when performing input/interruption of the power source thereof. In other words, there is a demand that at the time of the power source input, the high voltage drive power source of the liquid crystal display rises at the same time as or later than the low voltage drive power source of the logic circuit. Also, at the interruption of the power source, it is demanded that the high voltage drive power source of the liquid crystal display falls earlier than or at the same time as the low voltage drive power source of the logic circuit.
However, in practice, it is difficult to fall the high voltage drive power source of the liquid crystal display faster than the low voltage drive power source of the logic circuit at the interruption of the power source. The reasons for this are enumerated as follows:
(1) Under the state in which the power source can be interrupted, because the drive of the image reading device, the motor, or the like for driving the movable parts such as the sheet feed roller, the photosensitive drum, and the fixed roller are not in operation, an amount of a consumed current of the power source circuit is small, and therefore the power source circuit is rendered into a stand-by mode;
(2) In addition, when the drive of the image reading device, the motor, or the like for driving the movable parts such as the sheet feed roller, the photosensitive drum, and the fired roller are in operation, power source capacity of the high voltage drive power source is made large so that ample power can be supplied to the motor or the like for driving the movable parts, and hence a voltage drop of the power source circuit at the time of power source interruption is slow; and
(3) Still further, because the electricity consumed by the low voltage drive power source of the logic circuit during the stand-by mode is almost equivalent to that during operation, the voltage thereof drops relatively faster at the time of power interruption.
Regarding a drive voltage control device of a liquid crystal display, there is one disclosed in, for example, Japanese Patent Application Laid-open No. Heisei 11(1999)-282427. The drive voltage control device of the liquid crystal display disclosed in this publication has a control circuit as shown in FIG. 13.
As shown in FIG. 13, the drive voltage control device of the liquid crystal display has a power source circuit 1, which outputs voltages such as a stand-by voltage (+5VE) and a drive voltage (+24V), as well as switching means 2 inserted in a drive voltage supply line. A control voltage (+5V) thereof and the drive voltage (+24V) are to the applied to the liquid crystal display.
The drive voltage control device of the liquid crystal display includes timing control means (timing circuit) made up of a reset circuit 3, flip-flop circuits 4 and 5, an inverter 6, resistors 7 and 8, or the like.
According to this timing control means, the aforementioned switching means 2 is in a conductive state after the rise of the aforementioned control voltage (+5V) when changing to a power-saving mode OFF and when changing to a power-saving mode ON, the aforementioned switching means 2 is in a nonconductive state at the point where the aforementioned control voltage (+5V) starts to fall. The control voltage (+5V) and the drive voltage (+24V) are thus outputted when the power-saving mode is OFF. Further, when the power-saving mode is ON, output of the control voltage and the drive voltage are stopped. It should be noted that reference symbol 1a denotes a main power source switch and reference symbol 1b denotes a switch which will be shut when in the power-saving mode OFF.
According to this structure, the control voltage (+5V) is applied to the control circuit of the liquid crystal display (LCD) and the drive voltage (VEE=+24V) is applied to the liquid crystal display (LCD) after the control circuit starts operating. Therefore, when the power source circuit 1 is switched to the power-saving mode OFF, the liquid crystal display (LCD) is not damaged by the drive voltage (VEE=+24V). Further, the drive voltage (VEE=+24V) is quickly interrupted when the control voltage (+5V) starts to fall, whereby when switching to the power-saving mode ON, the drive voltage (VEE=+24V) is not maintained for a long time even after the operation of the control circuit stopped. As a result, the liquid crystal display (LCD) is not damaged.
However, in the drive voltage control device of the liquid crystal display such as the one shown in FIG. 13, the discharge of the residual charge inside the LCD during power source interruption is slow. Therefore, the fall of the LCD drive power source may possibly be delayed during power source interruption.
For example, when a relatively large number of capacitors are provided inside the LCD (liquid crystal display) for the purpose of enhancing the display quality thereof, the discharge of the residual charge inside the LCD (liquid crystal display) is delayed even though the supply of electricity to the LCD is cut off during power source interruption. As a result, the fall of the LCD drive power source may possibly be delayed accordingly. In this case, a problem of preventing the liquid crystal of the liquid crystal display from deteriorating during power source interruption of the LCD power source is still not solved.
The present invention has been made to solve such a problem, and therefore has an object to provide an LCD power source control method in which a residual charge inside a liquid crystal display during power source interruption can be positively removed and power source voltage supply/interruption of each of a logic circuit and a drive circuit can be performed in a correct order with a simply structured circuit, whereby deterioration of the liquid crystal display is minor. Further, the present invention has other objects to provide a control circuit and an image forming apparatus having the control circuit used in the LCD power source control method.
To achieve the above objects, in the LCD power source control method of the present invention, a voltage of a logic circuit power source in a module is detected by way of voltage detecting means to thereby perform supply/interruption of a voltage from an LCD drive power source to an LCD control circuit by means of LCD power source supply/interruption means. Meanwhile, the residual charge of the LCD drive circuit is forcibly discharged by means of compulsory discharge means when the LCD power source supply/interruption means is performing interruption. Further, in the LCD power source control method, upon detecting a voltage drop of the logic circuit power source, the voltage detecting means immediately outputs a signal indicating the voltage drop, whereby together with causing the LCD power source supply/interruption means to be in an interruption state, the compulsory discharge means is caused to be in an operation state to thereby control a discharge so that the residual charge of the LCD drive circuit is forcibly discharged by means of the compulsory discharge means before the voltage of the logic circuit power source becomes 0V.
According to the LCD power source control method, the compulsory discharge of the residual charge of the liquid crystal display can be instantly terminated in the period between times when the voltage detecting means detects the voltage drop of the logic circuit power source and when the circuit drive voltage becomes 0V.
According to this result, for example, even if a relatively large number of capacitors are provided inside the LCD (liquid crystal display) for the purpose of enhancing the display quality thereof, the discharge of the residual charge inside the LCD is performed instantaneously and forcibly when the supply of electricity to the LCD is interrupted at the time of power interruption. As a result, the fall of the LCD drive voltage (liquid crystal display drive voltage) is instantaneously performed so that the LCD drive voltage can become 0V before the logic circuit voltage becomes 0V. Therefore, the reverse flow of a current from the drive circuit of the liquid crystal display to the logic circuit when the operation of the liquid crystal display is OFF, that is, when the LCD power source supply/interruption means is performing interruption is prevented. The destruction of the logic circuit and the liquid crystal display when the operation of the liquid crystal display is OFF is thus prevented.
Further, in the LCD power source control method, upon detecting a voltage rise of the logic circuit power source, the voltage detecting means can be rendered to delay the output of a signal of the voltage rise for a fixed time until the voltage of the logic circuit power source becomes stable at a predetermined voltage, whereby together with causing the LCD power source supply/interruption means to a power supply state, the compulsory discharge means is caused to be in an open state.
According to the LCD power source control method, when the operation of the liquid crystal display is ON, that is, when the LCD power source supply/interruption means is supplying the power source, the reverse flow of a current from the drive circuit to the logic circuit of the liquid crystal display is prevented. The destruction of the logic circuit and the liquid crystal display when the operation of the liquid crystal display is ON is thus prevented.
Still further, the LCD power source control circuit implementing such a method includes: a plurality of power sources structured so that 2 power sources or more are supplied, having at least the logic circuit power source in a module and the LCD drive power source; voltage detecting means for detecting a voltage of the logic circuit power source; LCD power source supply/interruption means for performing supply/interruption of a voltage from the LCD drive power source to the LCD control circuit; and compulsory discharge means for forcibly discharging a residual charge of an LCD drive circuit when the LCD power source supply/interruption means is in interruption, wherein the voltage detecting means can be formed to have a structure in which the voltage detecting means immediately outputs a signal indicating a voltage drop upon detecting the voltage drop of the logic circuit power source, whereby together with causing the LCD power source supply/interruption means to be in the interruption state, the compulsory discharge means is caused to be in the operation state to thereby forcibly discharge the residual charge of the LCD drive circuit by means of the compulsory discharge means before the voltage of the logic circuit power source becomes 0V.
Further, upon detecting a voltage rise of the logic circuit power source, the voltage detecting means of the LCD power source control circuit delays the output of the signal indicating the voltage rise for a fixed time until the voltage of the logic circuit power source becomes stable at a predetermined voltage, whereby it is possible to control the LCD power source supply/interruption means to be in the power supply state, and the compulsory discharge means in the open state.
Further, the LCD power source control circuit can be structured to have discharge electric current restriction means provided therein to prevent a large current from flowing between the LCD power source supply/interruption means and the compulsory discharge means when both means are in operation at the same time. According to this structure, a maximum discharge electric current value flowing in the compulsory discharge means can be set in accordance with the amount of the residual charge of the liquid crystal display to be used because the discharge electric current restriction means is provided therein. In addition, protection of the compulsory discharge means can be performed. In other words, when the residual charge of the liquid crystal display is discharged by means of the compulsory discharge means, the discharge electric current restriction means restricts the current flowing in the compulsory discharge means so that the compulsory discharge means is not destroyed, whereby damage to the compulsory discharge means owing to the forced discharge of the residual charge can be prevented beforehand.
Still further, the LCD power source supply/interruption means can include a drive switching element connected to the LCD power source, which is switched to the interruption state due to a signal indicating that the voltage detecting means detects a voltage drop, and which is switched to a discharge state due to a signal indicating that the voltage detecting means detects a voltage rise. In addition, the compulsory discharge means can include a discharge switching element connected to earth, which is switched to the operating state due to a signal indicating that the voltage detecting means detects a voltage drop, and which is switched to the open state due to a signal indicating that the voltage detecting means detects a voltage rise.
According to this structure, the supply and interruption of the LCD power source to the LCD drive circuit can be performed with a simple structure, and the discharge of the residual charge of the LCD drive circuit can also be performed with a simple structure.
Further, the discharge electric current restriction means can be a resistor connected in series between the LCD power source supply/interruption means and the compulsory discharge means. According to this structure, a resistor for restricting a discharge electric current is provided therein, and therefore a resistance value is set in accordance with the amount of the residual charge of the liquid crystal display to be used, whereby a maximum discharge electric current value flowing in the compulsory discharge means can be simply and easily set. In addition, protection of the compulsory discharge means can be performed. In other words, when the residual charge of the liquid crystal display is discharged by means of the compulsory discharge means, the discharge electric current restriction resistor restricts the current flowing in the compulsory discharge means so that the compulsory discharge means is not destroyed, whereby damage to the compulsory discharge means owing to forced discharge of the residual charge can be prevented beforehand.
Still further, the LCD power source supply/interruption means can be formed of a first control switching element, which is connected to earth, and a plurality of resistors in addition to the drive switching element. According to this structure, ON/OFF control of the drive switching element can be easily performed by the first control switching element and the plurality of resistors.
Further, the compulsory discharge means can be provided with a second control switching element, which is connected to a control logic circuit power source, and a plurality of resistors in addition to the discharge switching element. According to this structure, the ON/OFF control of the discharge switching element can be easily performed by the second control switching element, which is connected to the control logic circuit power source, and the plurality of resistors.
Still further, the drive switching element, the discharge switching element, and the first and second switching elements can be transistors that have the above-mentioned respective characteristics.
Further, the discharge switching element can be a MOS FET also serving as the discharge electric current control resistor due to an internal resistor. According to this structure, when performing forced discharge of the residual charge of the liquid crystal display to be used, the maximum discharge electric current value flowing in the discharge MOS FET can be set in accordance with the amount of the residual charge of the liquid crystal display to be used by selecting the MOS FET which has an internal resistor that is not destroyed by the current flowing therein. In addition, protection of the discharge MOS FET can be conducted. Furthermore, means to forcibly discharge the residual charge of the liquid crystal display can be structured at a low cost. Moreover, the structure of the compulsory discharge means can be simplified by reducing one of the components compared with the one formed of the discharge switching element and the discharge electric current restriction means.
Further, the voltage detecting means can be set so that it judges that a voltage drop has been detected when a voltage VCC of the logic circuit power source becomes lower than a predetermined threshold value and judges that a voltage rise has been detected when the voltage VCC of the logic circuit power source becomes higher than the predetermined threshold value.
Further, the voltage detecting means can be set so that the signal indicating a voltage rise is not immediately outputted but delayed for a fixed time, whereby the signal is outputted after the voltage of the logic circuit power source is stable at a voltage of operation.
In this case, the reverse flow of a current from the drive circuit of the liquid crystal display to the logic circuit when the operation of the liquid crystal display is ON is prevented. As a result, the destruction of the logic circuit and the liquid crystal display when the operation of the liquid crystal display is ON is prevented.
Further, the voltage detecting means can be structured to share a reset circuit for resetting the logic circuit and control means thereof when the voltage detecting means has detected a voltage drop of the logic circuit power source and releasing the reset when a voltage rise has been detected so that the control means of the logic circuit does not go out of control.
Still further, a fax that has a copying function and a printing function or a copying machine (image forming apparatus) that has a data transmission function, a copying function, and a printing function can be structured to have the above-mentioned LCD power source control circuit.